<p>Electrochemical Passivation and Morphological Characterization of Tungsten Anodic Films in Different pH Environments</p>

This study investigates how electrolyte pH and applied anodic potential influence the qualitative electrochemical response and surface-visible morphology of tungsten anodic films. Four aqueous electrolyte conditions that represented strongly acidic, neutral, mildly alkaline, and strongly alkaline conditions were used to anodically polarize the tungsten working electrodes (0.25 x 20 mm exposed dimension): 0.1 M H2SO4 (pH 1), phosphate buffer (pH 7), phosphate buffer (pH 9), and 0.1 M NaOH (pH 13). An anodic response region was identified using potentiodynamic polarization, and scanning electron microscopy (SEM) was used to examine the resulting surface morphology at selected potentials. The results of the SEM analysis indicate that pH and applied potential jointly influence surface-observable characteristics. Rougher, porous-appearing regions of the surface were more apparent in strongly acidic and strongly alkaline media. Approximate surface-opening sizes included 125-135 nm at pH 1 and 0.75 V, 126-300 nm at pH 9 and 0.275 V, 200-700 nm at pH 9 and 0.4 V, and 105-600 nm at pH 13 and selected potentials. However, the interpretation of polarization curves is based solely on qualitative considerations because the current data do not permit Tafel analysis, electrochemical impedance spectroscopy, gas-sensing tests, oxide-thickness measurements, X-ray Diffraction, or X-ray Photoelectron Spectroscopy​. Thus, the results can be interpreted as indicating pH- and potential-dependent tungsten surface modification, but not as a direct validation of the presence of methane in the air. The research is relevant because the ability to control tungsten surface morphology is an essential preliminary step for applications that require surface activity, such as future tungsten-based sensing and electrochemical-device optimization.